An imaging apparatus such as a copier and printer is generally designed to form an electrostatic latent image on a photosensitive drum which is a latent image carrier using a laser beam. Such an apparatus modulates a laser beam oscillated by a laser oscillator according to an image signal and irradiates the laser beam onto a polygonal mirror rotating at a high speed. This laser beam is reflected by the polygonal mirror, deflected and scanned along an axial direction of the photosensitive drum as it rotates. This laser beam forms an image on the photosensitive drum through an image forming lens, forming an electrostatic latent image according to each scanning line of the image.
This type of image forming apparatus is required to form a high resolution image from the standpoints of reproducibility and proportion or the like of fine characters and, for example, many apparatuses whose image forming unit has a resolution of 1200×1200 dpi or above are being realized. Furthermore, there is a proposal as described in Japanese Patent Laid-Open No. 6-161195 of controlling an amount of laser exposure and setting conditions of the apparatus according to image information (attribute) to maintain image quality of plane image (tone image) and line image (character and line image).
However, in the case of the above described conventional technology, if tone images, characters and line images are mixed in the same image, it is quite difficult to realize image qualities of the tone image and characters satisfactorily. Especially, when resolution of an image forming apparatus is increased, the difference between both image qualities becomes more noticeable.
More specifically, when plane image (tone image) forming conditions are set over an entire image, the line image (character and line images) in the image fattens, and therefore crushing of fine characters in particular, that is, deterioration of resolution occurs. Furthermore, when line image (character and line image) forming conditions are set over the entire image, a maximum optical density of the plane image (tone image) in the image decreases. In a full color image forming apparatus in particular, its reproducible color range decreases and a satisfactory image cannot be obtained.
Moreover, when the resolution is increased, it is not possible to solve the problem that the reproducibility of the low density area of the tone image becomes unstable. FIG. 12A shows a schematic view of a latent image profile when an electrostatic latent image of a 1-pixel, 100% image is formed on an image carrier with a resolution of 600×600 dpi. FIG. 12B shows a schematic view of a latent image profile when an electrostatic latent image of a 1-pixel, 100% image is formed on an image carrier with a resolution of 1200×1200 dpi. Here, an integrated amount of laser beam at each apparatus is constant so that both optical densities become the same. As is obvious from this figures, when a latent image is formed with a higher resolution (FIG. 12B), the electrostatic latent image is shallow and more widely distributed.
Examples of factors that an optical density of image changes include a variation of electric charge potential of the image carrier (photosensitive drum), variation of potential of the exposure unit, variation of developing bias and variation of electric charge of a developer.
FIG. 13A to FIG. 13F relatively illustrate such variation factors of optical density of image using a variation of developing bias.
FIG. 13A to FIG. 13C depict a potential distribution 1002 of an electrostatic latent image per pixel of a 1-pixel, 100% image with a resolution of 600×600 dpi. On the other hand, FIG. 13D to FIG. 13F depict a potential distribution 1002 of an electrostatic latent image per pixel of a 1-pixel, 100% image with a resolution of 1200×1200 dpi. Reference numeral 1001 denotes a developing bias level and a portion 1003 below this developing bias level 1001 is the development portion which is developed (having an area proportional to the optical density) and is equivalent to the amount of toner adhered.
In FIG. 13A and FIG. 13D, the developing bias 1001 is substantially a reference value and the area demarcated by this developing bias 1001 corresponds to the amount of toner, that is, optical density and both have substantially the same optical density in this condition. On the other hand, as shown in FIG. 13B and FIG. 13E, when the developing bias level 1001 is raised, the optical density when an image with resolution 1200 dpi is formed (FIG. 13E) is greater than the optical density when an image with resolution 600 dpi is formed (FIG. 13B) and the size of the pixel (dot) with resolution 1200 dpi (FIG. 13E) is greater. On the contrary, as shown in FIG. 13C and FIG. 13F, when the developing bias level 1001 is lowered and the development contrast is decreased, the optical density when the image with resolution 600 dpi is formed (FIG. 13C) is greater than the optical density when the image with resolution 1200 dpi is formed (FIG. 13F). Thus, it is appreciated that when compared with the case with resolution 600 dpi, the optical density becomes unstable when an image is formed with higher resolution 1200 dpi.
When, for example, an image pattern as shown in FIG. 6 is formed under such a condition, the uniformity of optical density in a halftone, highlight area in particular, deteriorates, resulting in a rough image with high perceived granularity because variations of optical density in different positions are large.
Furthermore, a technique for stabilizing a latent image by changing halftone processing and reducing the apparent number of lines is also proposed and realized. However, little effect is obtained with regard to the highlight area until the latent image is stabilized, that is, the image data is increased to a certain degree as in the case of the above explanation. This is because even if the processing is changed and the number of the lines is reduced, the basic resolution of image data itself cannot be changed (reduced) and as a result, a shallow, wide electrostatic latent image is formed in the highlight area. That is, even when conventional technologies are combined, it has been impossible to achieve all of stability of optical density of the tone image within an identical image, highlight area in particular, smoothness, improvement of resolution of a character/line image and uniformity of proportion as the level of resolution of an image forming apparatus increases.